Experiments conducted below 0.1 K require expensive, complicated cryogenic apparatus. What if an integrated solid-state device could be used instead? The authors present just such an on-chip refrigerator, based on a superconductor/normal-metal junction, that chills down to 30 mK with remarkable cooling power. This provides a means to reduce thermal noise in e.g. qubits, SQUIPT magnetometers, or the sensitive detectors required for astronomy experiments.

Platinum is a favorite material for numerous applications, including electrodes for solid oxide fuel cells, but of course it is costly. The authors use dealloying to prepare thin films of spongelike nanoporous Pt-Y-Al, the yttrium being added to engineer the material’s bandstructure and reduce the amount of platinum required. This yields electrodes with enhanced thermal stability and 13 times the electrocatalytic activity of conventional systems.

A new and surprising property is discovered when a ferroelectric nonvolatile gate is combined with a conductive oxide channel: When the polarization is switched, a single atomic layer in the normally insulating ferroelectric becomes conductive. This layer has a high mobility and becomes the dominant conductive channel in the all-oxide heterostructure. This approach can be extended to control the properties of any single atomic layer at the interface between a ferroelectric and a channel material, and presents a qualitative shift in our understanding of the ferroelectric field effect.

When it comes to spinning polymer fibers, spiders are much better than scientists. However, the authors have narrowed the gap: Extensive simulations show that for by judiciously oscillating the spinneret, instabilities can be tamed and extremely thin fibers can be extracted. Remarkably, this effect is independent of the rheology of the polymeric solution used. These ultrathin fibers open up a new length scale for applications ranging from photonics and organic field-effect transistors to artificial ligaments and scaffolds used in tissue culture.

Remote sensing of trace gases in the atmosphere is used to detect toxins, monitor pollution, and verify treaties, but presently is limited when it comes to recognizing and discriminating similar chemicals. The authors quantitatively assess the potential for a technique with high recognition specificity, even among isotopic isomers (isotopomers), at distances up to 1 km.

A new and surprising property is discovered when a ferroelectric nonvolatile gate is combined with a conductive oxide channel: When the polarization is switched, a single atomic layer in the normally insulating ferroelectric becomes conductive. This layer has a high mobility and becomes the dominant conductive channel in the all-oxide heterostructure. This approach can be extended to control the properties of any single atomic layer at the interface between a ferroelectric and a channel material, and presents a qualitative shift in our understanding of the ferroelectric field effect.

Experiments conducted below 0.1 K require expensive, complicated cryogenic apparatus. What if an integrated solid-state device could be used instead? The authors present just such an on-chip refrigerator, based on a superconductor/normal-metal junction, that chills down to 30 mK with remarkable cooling power. This provides a means to reduce thermal noise in e.g. qubits, SQUIPT magnetometers, or the sensitive detectors required for astronomy experiments.

Magnons are quantized quasiparticles that can in principle be used in quantum computation. To implement such computations in practice, magnons must be strongly coupled with photons, which transfer information between them. In this work, the authors demonstrate extremely strong couplings using a type of multipost microwave cavity that can focus a magnetic field into submillimeter-sized samples. This ultrastrong coupling of magnons and photons can be a building block in the architecture of high-fidelity hybrid quantum systems for the processors of the future.

The authors present an effective-medium theory of the optical diamagnetic response of very dilute metal colloids. These dispersions of ferromagnetic metal clusters in diamagnetic hosts show a large, linear response under an applied magnetic field, which is of interest for emerging applications in sensing, integrated optical communications, and magneto-optical current transformers and transducers. This theory could be further extended to describe e.g., metal inclusions in polymers or glasses.

Tailoring the microscopic structures of oxide interfaces and surfaces to realize desired electronic properties is of intense interest. The authors modulate the conductivity of LaNiO${}_3$ films as thin as three unit cells by precisely controlling the composition of the topmost atomic layer. Three-dimensional x-ray imaging combined with first-principles theory exposes the correlation between surface termination and electrical conductivity, which could be widely exploited in logic, memory, or sensor applications.

Hydrogenated amorphous silicon is a technologically important material, vital to applications as varied as photovoltaics, thin-film transistors, and “night vision” goggles. All of these applications rely on the function of dopants, but, remarkably, our understanding of how the most common dopants infiltrate into a material to perform crucial electronic functions remains largely empirical. The authors provide a comprehensive ab initio study of two key dopants in amorphous silicon, including lattice dynamics and hydrogen hopping and passivation. This insight provides a firm foundation for rational optimization of practical semiconductors.

The authors explore the feasibility of ultrahigh-frequency devices by utilizing the unique features of two-dimensional (2D) crystals. These materials range from gapless semimetals to wide-band-gap insulators, and a number of them ($e.g.$ hexagonal boron nitride and transition metal dichalcogenides) can be integrated seamlessly with graphene, without causing defects, interfacial scattering centers, or degradation of properties. A detailed theoretical analysis indicates the potential for 2D crystal heterostructures that could process information at rates well over a terahertz–several times the current technological limit.

Metamaterials can be engineered to control not just light, but also acoustic waves. The authors fabricate a class of three-dimensional, linear elastic metamaterials for which the effective bulk modulus and mass density can be adjusted independently over a large range. This tuning is not possible in ordinary materials, but through geometric design this scheme allows for “acoustic cloaking”, in which an object may be shielded from noise or other sound waves, such as sonar.

Graphene adheres very strongly to solids and is impermeable to many gases, which allows the formation of suspended graphene blisters of controllable shape. The authors use micro-Raman spectroscopy to perform contactless measurement of the topography and stress/strain parameters of these blisters. This unique “all-optical blister test” can be generalized to other systems and invites applications in pressure sensing and nanoelectromechanical systems (NEMS).

A current motif in physics research is the use of strain to tailor the electronic properties of a material. One well-defined source of strain is the lattice mismatch at the interface of two crystals, such as GdN (a ferromagnetic insulator) and GaN (a direct-band-gap semiconductor). The authors calculate that such an interface under strain should cause GdN to become a half-metal and to form a Schottky barrier with GaN. This could permit strained interfaces to act as rectifiers in spintronics or other applications.

A key feature of a topological insulator is the correlation of spin and propagation direction of electrons at its surface: a potentially valuable property for spintronics applications. The authors show how this feature can be used in tunneling magnetoresistance (TMR) devices that would have similar or better TMR ratios than traditional devices, even at room temperature; that would require only one ferromagnetic layer, not two; and that could be used to measure the spin polarization of the topological surface state.

When it comes to spinning polymer fibers, spiders are much better than scientists. However, the authors have narrowed the gap: Extensive simulations show that for by judiciously oscillating the spinneret, instabilities can be tamed and extremely thin fibers can be extracted. Remarkably, this effect is independent of the rheology of the polymeric solution used. These ultrathin fibers open up a new length scale for applications ranging from photonics and organic field-effect transistors to artificial ligaments and scaffolds used in tissue culture.

Atom interferometers use light to track the Doppler effect as ensembles of cold atoms such as ${}^{87}$Rb travel ballistically in vacuum. These systems are used as ultrasensitive gravimeters and could also be exceptional broadband inertial sensors for vehicle navigation and guidance, but typically they are designed for a static laboratory environment. The authors present a compact atom interferometer that measures acceleration and rotation simultaneously and at a high rate, to be used in a dynamic environment for real-time integration of a vehicle’s equations of motion.

Generating electricity from a temperature gradient (due to waste heat from an engine or power plant, say) is seen as a significant aspect of the energy economy. Here the authors propose a thermoelectric device that takes advantage of bilayer excitons, electron-hole bound states that can form at an interface. By enhancing both thermopower and electrical conductivity, the counterflow construction of such bilayer-exciton systems can increase the thermoelectric figure of merit by an order of magnitude, compared to that of a bulk material.

Spin-labeling with paramagnetic ions is important in determining the structures of biomolecules, which are generally large and complex, but current techniques lack the sensitivity to detect a few isolated spins. The authors use a nitrogen-vacancy (NV) center in diamond to image nanoscale volumes of paramagnetic gadolinium compounds on the tip of an atomic force microscope. This is an important step toward imaging isolated spin-labeled molecules.

Platinum is a favorite material for numerous applications, including electrodes for solid oxide fuel cells, but of course it is costly. The authors use dealloying to prepare thin films of spongelike nanoporous Pt-Y-Al, the yttrium being added to engineer the material’s bandstructure and reduce the amount of platinum required. This yields electrodes with enhanced thermal stability and 13 times the electrocatalytic activity of conventional systems.

Remote sensing of trace gases in the atmosphere is used to detect toxins, monitor pollution, and verify treaties, but presently is limited when it comes to recognizing and discriminating similar chemicals. The authors quantitatively assess the potential for a technique with high recognition specificity, even among isotopic isomers (isotopomers), at distances up to 1 km.